1. Field of the Invention
The present invention relates to an image-forming optical system and, more particularly, to an image-forming optical system using a decentered optical system having a power, which comprises a decentered reflecting surface.
2. Discussion of Related Art
There has heretofore been known a compact reflecting decentered optical system as disclosed in Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 59-84201. This is an invention of a one-dimensional light-receiving lens comprising a cylindrical reflecting surface; therefore, two-dimensional imaging cannot be effected with the conventional optical system. JP(A) 62-144127 discloses an optical system wherein the identical cylindrical surface is used twice to effect reflection in order to reduce spherical aberration in the above-mentioned invention. JP(A) 62-205547 discloses the use of an aspherical reflecting surface as a reflecting surface, but makes no mention of the configuration of the reflecting surface.
U.S. Pat. Nos. 3,810,221 and 3,836,931 both disclose an example in which a rotationally symmetric aspherical mirror and a lens system having a surface which has only one plane of symmetry are used to constitute a finder optical system of a reflex camera. In this example, however, the surface having only one plane of symmetry is utilized for the purpose of correcting the tilt of a virtual image for observation.
JP(A) 1-257834 (U.S. Pat. No. 5,274,406) discloses an example in which a surface having only one plane of symmetry is used for a reflecting mirror to correct image distortion in a rear projection type television. In this example, however, a projection lens system is used for projection onto a screen, and the surface having only one plane of symmetry is used for correction of image distortion.
JP(A) 7-333551 discloses an example of a back-coated mirror type decentered optical system using an anamorphic surface and a toric surface as an observation optical system. However, the decentered optical system is not sufficiently corrected for aberrations, including image distortion.
None of the above-described prior art references a surface having only one plane of symmetry as a back-coated mirror to form a folded optical path.
Other examples of conventionally known optical systems include those disclosed in JP(A) 9-258105, 9-258106, 9-222561, 8-292372, 8-292368 and 8-248481. In these conventional optical systems, image formation takes place in the course of travel of light along the optical path. This causes the optical path to lengthen, making it impossible to construct a compact variable-magnification optical system.
Other examples are disclosed in JP(A) 9-222563, 9-211330, 9-211331 and 8-292371. However, these optical systems cannot change the magnification.
In the conventional rotationally symmetric optical systems, a transmitting rotationally symmetric lens having a refracting power is assigned to exert the required refracting power. Therefore, many constituent elements are needed for aberration correction. In the conventional decentered optical systems, an imaged figure or the like is undesirably distorted and the correct shape cannot be recorded unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
In a rotationally symmetric optical system comprising a refracting lens which is formed from a surface rotationally symmetric about an optical axis, a straight-line optical path is formed. Therefore, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large structure.